As is known in the art, rectifiers can be used to receive AC (alternating current) input signals and provide DC (direct current) output signals to energize a DC load. Rectifiers use one or more diodes to convert AC signals to DC signals. Half-wave rectifiers typically use one diode to block one-half of the AC waveform and pass the other half of the waveform. Multiple diodes can be used to provide full-wave rectification in which both the positive and negative cycles of the AC waveform are rectified. The full wave rectifier converts the input waveform to a DC output signal having a constant negative or positive polarity by reversing the negative (or positive) portions of the alternating current waveform.
For applications having three-phase operation, such as high voltage applications, six diodes can be used to provide the rectifier. Generally, three diode-pairs are used, one pair for each phase. For high voltage application, power factor correction may be required. Power factor correction can be provided using semiconductor switches to control conduction of the diodes. As the power ratings increase, the required voltage rating for semiconductor switches also increases. However, the voltage rating of the rectifier may be limited by the voltage rating of available switches used for power factor correction.
FIG. 1 shows a prior art high voltage rectifier 10 having power factor correction in an arrangement referred to in the art as a Vienna rectifier. Three phases A, B, C are provided to a diode bridge 12 to provide a DC output signal on first and second output capacitors Cload1, Cload2. Respective switches Q1, Q2, Q3, for each phase A, B, C switch the phases in a manner well known to one of ordinary skill in the art in accordance with operational requirements of the Vienna rectifier. If the desired DC output is 24,000 Volts, for example, each of the switches Q1, Q2, Q3, must be rated for at least 12,000 Volts. At present, practical switches carrying such voltages are not available.